The present invention relates to wireless communication systems, and more particularly, to a method and an apparatus for automatic gain control compensation in a wireless communication system.
Wireless communication systems that utilize digitally coded communication signals are known in the art. One such system is a direct sequence Code Division Multiple Access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 2000 (TIA IS-2000) herein after referred to as IS-2000. In accordance with IS-2000, the coded communication signals used in the DS-CDMA system include signals that are transmitted in a common channel, typically a 1.25 MHz bandwidth common channel, between mobile stations (MS) and base transceiver stations (BTS), or base stations, located at the base sites (BS) of a wireless communication system. A BTS transmitting to a mobile station is often referred to as a serving BTS.
Another such system is a Universal Mobile Telecommunications System (UMTS) cellular communication system, such as set forth in IMT-2000. Like the IS-2000 system, UMTS is a DS-CDMA based technology that transmits signals in a common channel between mobile stations and BTS""s.
A digital wireless communication system is a complex network of elements and their interconnections and protocols. Typically elements include (1) a radio link to the mobile stations (e.g., cellular telephones), which is provided by one or more base transceiver stations, (2) communication links between the base stations, (3) a controller, typically one or more base station controllers or centralized base station controllers (BSC/CBSC), to control communication between and to manage the operation and interaction of the base stations, (4) a call controller or switch, typically a call agent (i.e., a xe2x80x9csoftswitchxe2x80x9d), for routing calls within the system, and (5) a link to the land line or public switch telephone network (PSTN), which is usually also provided by the call agent.
During a typical wireless communication, the mobile station communication signal is supported by the BTS associated with the coverage area in which the mobile station is traveling. Each BTS coverage area may include one or more cells depending on the configuration of the wireless communication system. Thus, the radio link supporting the mobile station communication signal may remain with one particular BTS even though the mobile station travels from cell to cell, or the radio link supporting the mobile communication signal may be handed-over to, or reestablished with, another BTS. In addition, the cell in which the mobile station is located at any point in time may be referred to as the same-cell while all other cells may be referred to as other-cells.
A coded communication signal transmitted by a serving BTS typically arrives at a mobile station having an information bearing signal portion and an interfering signal portion. The information bearing signal portion contains the originally transmitted information. The interfering signal portion may or may not contain the originally transmitted information. The interfering signal portion may include two interference components, same-cell interference and other-cell interference. Same-cell interference is created by multipath reflections of transmissions from those BTS(s)/cells serving the mobile station and may vary considerably in power over a short time. Other-cell interference is generated by other BTS(s)/cells and has a relatively constant signal power.
Although other-cell interference has a relatively constant signal power, the power of the coded signal received at the mobile station from the BTS may vary dramatically depending on the proximity of the mobile station to the base station and the transmitter power of the BTS. In addition, power of the coded signal received at the mobile station from the BTS may vary as a result of multipath fading due to reflections and/or scattering of the signal off of nearby scatters such as buildings. Accordingly, a mobile station receiver having, among other things, an analog-to-digital converter (ADC), a demodulator, and a decoder, must be designed to receive signals that vary over a wide range of signal power.
In general, the mobile station receiver utilizes an automatic gain control (AGC) circuit to cause an adjustment to the amplification of the coded signal received at a mobile station antenna in an attempt to compensate for the wide range of signal power. In particular, the coded signal received by the mobile station antenna is adjusted by the AGC to some predetermined power level suitable for input into the ADC. In other words, the AGC receives the coded signal, performs an adjustment to the coded signal, and then outputs a post-AGC signal at a constant power level suitable for input to the ADC thereby allowing for an ADC with a fewer number of quantized levels than would be required without an AGC circuit. As a consequence of using an ADC with fewer quantized levels, the post-AGC circuit design is simplified because fewer bits are needed to represent signals in the demodulator, decoder, etc. The post-AGC signal includes, among other things, an interfering signal portion at an associated power level and an information-bearing signal portion at an associated power level.
Although operation of the AGC generally enables simplification of the overall post-AGC circuitry of the mobile station receiver, addition of the AGC to the mobile station receiver may require that a more complicated receiver demodulator design be used when a particular type of channel coding is utilized. For example, when convolutional coding is used such as the type used for coding a DS-CDMA information-bearing signal, the mobile station receiver demodulator is required to supply information to the mobile station receiver channel decoder. The information is calculated by the demodulator using the post-AGC signal and is equal to a ratio of the information-bearing signal power to the interfering signal power (SIR) of the sequence of received symbols of the information-bearing signal.
The ease with which the demodulator can calculate and supply the SIR information required by the decoder depends on whether the post-AGC interfering signal power level output varies. If the post-AGC interfering signal power level is almost constant, the demodulator can easily supply the required SIR information. If the post-AGC interfering signal power level output from the AGC is not constant, however, the demodulator is required to first estimate an interfering signal power level for each received symbol, and second, the demodulator must divide an estimated information-bearing signal power level for each received symbol by the estimated interfering signal power level for each received symbol to obtain the SIR of the sequence of received symbols of the information-bearing signal required by the decoder. As is known, the step of dividing the estimated information-bearing signal power level by the estimated interfering signal power level is computationally prohibitive.